Controlling the flow of inlet combustion air to a furnace or stove is difficult, especially for hand fired stoves or furnaces burning solid fuels such as wood or coal. Too much combustion air makes the fire overheat and burn the fuel too rapidly, and too little combustion air can make the fire go out. A steady flow of combustion air is best for even burning and heating; but draft and fire conditions vary widely, making this difficult to attain.
Another problem with inlet air control is the possibility of a chimney fire. If chimney deposits ignite, the draft increases enormously. Air rushes in through the stove's inlet port to feed the fire in the chimney, which can be very dangerous indeed.
Present regulators for the flow of combustion air into hand fueled stoves and furnaces are generally bimetallic coils that operate a damper valve over the inlet air port. Bimetallic coils cannot be mounted in the fire box to respond directly to fire temperatures, because they cannot survive the heat and combustion products there; so they are mounted outside the fire box where they are slow to react to changes in fire conditions. They do not respond to the heat of a chimney fire, and they do not prevent chimney fires from burning out of control. They are also difficult to adjust to satisfactory operation and are often inaccurate.
Fire temperature has a predominant influence on the draft of hot air and gasses up the flue or chimney, and this in turn influences the inflow of combustion air. But other factors including outside air temperature, humidity, and wind velocity also have large and varying effects on the flue draft rate and the combustion air intake. Opening the door to the fire box quickly lowers the fire temperature and allows room air to escape up the flue. This practically stops the flow of air through the inlet port while the fire box door is open, and the cooler fire then needs more combustion air after the fire box door closes. Adding fresh fuel immediately cools the fire down and requires extra combustion air to get the new fuel burning. Many other variations occur from the quality and amount of fuel in the fire box, circulation of air around the fire box, and other variables. Bimetallic inlet air controllers respond only sluggishly to these conditions and tend to delay necessary corrections long beyond the needs of the fire. They are also unable to respond to a chimney fire.
I have devised a control that regulates inlet combustion air accurately, continuously, and responsively to the needs of the fire in a solid fuel stove or furnace. My device can be adjusted to maintain different burn rates and operates to compensate rapidly as fire and air flow conditions vary. By continuously regulating the rate of air inflow, my device prevents chimney fires from getting started by blocking the excess air flow that is required to let a chimney fire ignite and flame. My device is simple, inexpensive, mountable on a wide variety of stoves and furnaces with different inlet ports, and capable of operating reliably and effectively in controlling inlet combustion air.